1. Field of the Invention
The present invention concerns a method of manufacturing wood material boards of the chip board type; the term wood material denotes all kinds of wood based components such as shavings, sawdust, peels and peeling refuse, cardboard and paper cutting refuse etc.
2. Prior Art
In conventional methods, these wood components are bonded into boards by means of heat hardened glue. The glues are usually so called synthetic resins that are introduced into the wood based raw material as precondensates. It is known also to use only urea to bond cellulosic pulp and wood components. At the same time urea has been found to reduce inflammability to various degrees depending on the amount of urea used. The advantage of the method employing urea in comparison with conventional methods is in the price of the bonding component and its simultaneous effect on inflammability and smoke formation.
The disadvantage of the prior art method employing urea is the slowness of the chemical reactions connected with the bonding stage. The pressing time required by this prior art method is approximately five-fold in comparison with that of the conventional technique. Now it has, however, been found that this pressing time may be greatly shortened by the method of the present invention.
With the exception of the pressing stage, the manufacturing process of wood material boards is usually continuous. The pressing stage is the "bottle neck" of the method, i.e. the stage that determines the speed of the preceeding and subsequent manufacturing stages. Therefore, steps that shorten the time required for pressing are technically and economically desirable.
The primary aim of the pressing stage is to bring the raw material of the board into the desired shape and to cause the bond forming chemical reactions to occur by means of heat applied through the pressing plates. Of these, the latter part, i.e. the transfer of activating energy required by the reactions into the board is the slowest and therefore the most influential factor on the overall pressing time.
Thus, the time needed in the pressing stage depends on both the amount of energy required to activate the reactions and the speed with which this energy can be transferred into the raw material. In principle, the activating energy for a chemical reaction, i.e. the rate coefficient of the reaction at a given temperature, can be affected catalytically. The speed with which this amount of heat energy is transferred into the board material may best be influenced by the difference in temperature between the pressing plates and the board material.
The relatively long pressing time required in the method employing urea according to our Finnish Patent Application No. 2097/71, is a result of the chemical nature of the method. The reactions forming bonds between wood particles are the result of many separate reactions, the total activating energy required by all reactions which is fairly great.
By means of the method of the present invention, the characteristics of which are defined in the appended claims, the pressing time in the board manufacture method employing urea as a bonding agent may be greatly shortened. This is achieved by reducing the total activating energy of the reactions by means of catalysts according to the present invention, whereby the rate coefficients of reaction, when using conventional pressing temperature in the method, are substantially increased, and secondly by causing the urea reactions to occur partially under the influence of heat before the urea is sprayed on the wood material, whereby the time required by these prereactions can be subtracted from the pressing time itself. The best result is achieved by a combination of these two methods whereby the reaction mixture is prepared from urea by heating it with the catalysts before spraying onto the wood material. The pressing time of the known method is from 1 to 10 minutes/board thickness in mm. In the method according to the present invention the pressing time is from 20 to 50 seconds/board thickness in mm.